Corn is expected to start showing signs of heat stress, especially corn planted late due to wet planting conditions. Extremely hot, dry weather is plaguing the Midwest this week.

The hot weather can cause extra problems in late-planted corn. Why? Corn planted in wet soils often developed poorer root systems, according to Ohio State Extension corn scientist Peter Thomison. This directly affects yield potential when dry weather occurs.

Because of the challenges in getting the crop in the ground, parts of the Midwest have had two crops: one planted early and one late after Memorial Day. Thomison says that the corn planted in mid-May is at or near tasseling now, while corn planted after Memorial Day is anywhere from stages V10 to V14. Some corn planted later in June may only be at the V6 stage.

"Given the variability of corn in the fields, some of the later corn isn't rooting very well because of soil conditions at planting — things like sidewall compaction and varying planting depths," Thomisons says. "Because of the intense heat and lack of rainfall this summer, some of that corn isn't performing well, even though it was planted in a moist soil bed. Some of this corn could be six feet tall, and in the same field there's corn waist high."

Along with the root structure and development issues, heat may affect yield potential by limiting the number of kernels developed. Thomison and his Extension colleagues have heard reports from some areas of Ohio about corn rolling relatively early each day and exhibiting moisture-related stress throughout the day.

While conventional wisdom says heat stress at pollination is a problem, Thomison says, if hot and dry conditions persist, the real problem this year will be kernel abortion instead of limited pollination.

"If we're looking at temperatures above 90 degrees sustained over a week, combined with soil moisture deficits, the impact on corn could be devastating," he reports. "These high temperatures alone may not jeopardize pollination, but in conjunction with water stress they can result in significant kernel abortion after pollination, during early grain fill. So, the ovules may get pollinated, but if we have inadequate moisture, the kernels will just abort and we'll see tip dieback."

He notes similar challenges in 2002, another year characterized by late plantings, where lack of moisture produced "nubbin ears" that significantly lowered yields. He sees potential yield loss of up to 5-10%/day if fields experience severe moisture stress in the two weeks before tasselling.

Recent windstorms throughout the eastern Corn Belt also caused problems because of the issues with root development. But Thomison says corn stalks for the most part have already bounced back. In most cases the wind damage was limited and localized, but some problems may still exist with corn stalk crimping, or pockets of green snap.

With expected rainfall totals at or below normal for the remainder of July and August, Thomison says he remains concerned about how well Mother Nature will treat Ohio farmers through season's end.

"The long-term forecast, to be brutally honest, is not promising," he said. "Some people will talk about maturity this year, but I'm more afraid the crop may just die of moisture stress in August."